The invention relates to a process for the biological purification of water by decomposing organic carbon with nitrate and molecular oxygen.
European patent application EP-A-0 247 211 discloses a generic process in which by connecting a nitrification stage with a denitrification stage in front of it, ammonium and also organic impurities in waste water can be decomposed by microorganisms. The biomass formed by the microorganisms is present in the form of a so-called biofilm, which covers the surfaces of the packing material of a fixed bed. The biofilm reactor of the first stage, in which denitrification occurs, has a heterogenous nature: In a partly confined vicinity there are regions in which aerobic and anoxic or anaerobic conditions prevail. In the aerobic regions the decomposition of the organic carbon is performed with the use of the dissolved molecular oxygen. In these regions there is no denitrification. This occurs in the anoxic regions, by the nitrate-containing oxygen being used by the release of molecular nitrogen (or other nitrogen compounds) by the biomass. Both types of oxygen use are possible for microorganisms of the same species; in the event of a deficiency of molecular oxygen these microorganisms adapt their metabolism by switching over to denitrification.
The biological decomposition of the ammonium is restricted by a large supply of organic carbon. Only after the elimination of the carbon performed in the first stage can the ammonium-containing nitrogen be oxidized, first into nitrite, and then into nitrate by nitrifying microorganisms (so-called nitric bacteria) in the ventilated biofilm reactor of the second stage. The nitrate is an intermediate product which is to be decomposed--by denitrification. A part of the nitrate-containing product water of the nitrification stage is returned to the first stage. As molecular oxygen is dissolved in the returned water because of the ventilation of the second state, an aerobic zone is produced in the entry region of the first stage, where denitrification occurs only to a limited extent. (As explained in more detail below by means of a Figure, an anoxic region where the desired denitrification takes place can also be formed in the presence of molecular oxygen.) If the supply of molecular oxygen is exhausted at some distance from the inlet point, then the full use of nitrogen-containing oxygen commences.
The organic carbon, i.e. the carbon which is contained in the biologically decomposable constituents of the waste water, is partly present in its dissolved form and partly in its immobile form enclosed in suspended solids. Only the dissolved organic carbon or the organic carbon which can be leached from the suspended solids is substantially susceptible to biological decomposition in the biofilm reactor. As the suspended solids are partly adsorbed on the biofilm, this impurity in the form of suspended solid particles or colloids is also eliminated.
The dissolved organic carbon is present in its easily decomposable and difficult to decompose form. The majority of the low-molecular alcohols (e.g. methanol or ethanol), acids (e.g. acetic acid), aldehyde and some more complex compounds (e.g. glucose) are easily decomposable; colloids, high-molecular compounds and humic acids decompose with more difficulty. When using nitrate-containing oxygen practically only the easily decomposable carbon can be eliminated. It is therefore important that the aerobic zone at the inlet region of the denitrification stage is kept minimal. The elimination of the more easily decomposable carbon is preferably also performed there, as a result of which the substrate suitable for the denitrification is partly lost.
In the known process denitrification is performed by means of a bioactive filter. With this reactor type the biofilm covers a granular base material, which in the form of ballast forms the fixed bed of the first reaction vessel. Together with the biological decomposition, the bioactive filter is also suitable for the removal of suspended solids; the suspended solids adsorbed on the base material and also a part of the growing biomass is removed as excess sludge from the purification process by a periodically performed flushing of the fixed bed by means of air and water.
In the second reaction vessel the fixed bed is constructed from a packing of laminar components having a regular geometrical structure (components of a static mixer). The nitric bacteria forming the biofilm have a far smaller growth than the microorganisms of the first stage; therefore rinsing is not required as often with the second stage.
The elimination of carbon should be performed as completely as possible in the first stage. At the same time as much nitrate as possible should be decomposed by using nitrate-containing oxygen. Only a deficiency of nitrate should be compensated by means of a supplementary supply of oxygen. If the ammonium content in the untreated water is not constant, there will be a variable supply of nitrate in the first stage and consequently the consumption of molecular oxygen will also be variable.